Hydroacoustics for the discovery and quantification of Nassau grouper (Epinephelus striatus) spawning aggregations

Abstract

Fish spawning aggregations (FSAs) are vital life-history events that need to be monitored to determine the health of aggregating populations; this is especially true of the endangered Nassau grouper (Epinephelus striatus). Hydroacoustics were used to locate Nassau grouper FSAs at sites on the west end of Little Cayman (LCW), and east ends of Grand Cayman (GCE) and Cayman Brac (CBE). Fish abundance and biomass at each FSA were estimated via echo integration and FSA extent. Acoustic mean fish abundance estimates (±SE) on the FSA at LCW (893 ± 459) did not differ significantly from concurrent SCUBA estimates (1150 ± 75). Mean fish densities (number 1000 m−3) were significantly higher at LCW (33.13 ± 5.62) than at the other sites (GCE: 7.01 ± 2.1, CBE: 4.61 ± 1.16). We investigate different acoustic post-processing options to obtain target strength (TS), and we examine the different TS to total length (TL) formulas available. The SCUBA surveys also provided measures of TL through the use of laser callipers allowing development of an in situ TS to TL formula for Nassau grouper at the LCW FSA. Application of this formula revealed mean fish TL was significantly higher at LCW (65.4 ± 0.7 cm) than GCE (60.7 ± 0.4 cm), but not CBE (61.1 ± 2.5 cm). Use of the empirical TS to TL formula resulted in underestimation of fish length in comparison with diver measurements, highlighting the benefits of secondary length data and deriving specific TS to TL formulas for each population. FSA location examined with reference to seasonal marine protected areas (Designated Grouper Spawning Areas) showed FSAs were partially outside these areas at GCE and very close to the boundary at CBE. As FSAs often occur at the limits of safe diving operations, hydroacoustic technology provides an alternative method to monitor and inform future management of aggregating fish species.

Notes

Acknowledgements

Thanks to all the staff of the Cayman Islands DoE for their assistance and making me so welcome and in particular, Bradley Johnson and Phillipe Bush, whose local knowledge resulted in the success of this research and also Laura Richardson and Jeremy Olynik for much general help. Thanks to the Reef Environmental Education Foundation and researchers associated with the Grouper Moon project for providing diver census data. Further, we thank Dr Helge Balk for advice on the data processing in Sonar5 and the comments of two anonymous reviewers whose input greatly increased the strength of the manuscript. The funding for the surveys was provided by The Darwin Initiative (UK Government) EIDPO045 (Assessing engagement in Cayman’s enhanced marine protected area system).

Supplementary material

Fig. S1Shadow effect analysis of echoes from within FSAs. a Four vertical analysis cells were defined in each FSA (indicated by the stack of rectangles). The x-axis shows depth (R(m)) and strength of signal return (dB), ping number is shown on the y-axis. b Mean Sv was calculated for each cell and compared for 15 separate acoustic detections of FSAs along the survey track from the top (1) to the bottom (4) of the FSAs. The lack of attenuation of echoes from deeper cells indicates absence of acoustic shadowing (TIFF 383 kb)